1. Field of the Invention
The present invention relates to thermal ink-jet printing and, more particularly, to inks for use in thermal ink-jet printers.
2. Description of the Prior Art
Thermal ink-jet printers, as is well known, employ a resistor element in a chamber provided with an orifice for ink to enter from a plenum, which in turn is connected to an ink reservoir. Typically, anywhere from 50 to well over 100 such resistor elements are arranged in a linear or rectangular array, thereby making up the print head. Each resistor element is associated with a nozzle and a nozzle plate through which the ink is expelled toward a substrate or print medium. The entire assembly of print head and reservoir comprise an ink-jet pen. In operation, a drop of ink is formed on the resistor surface. Heating of this drop by the resistor leads to explosive bubble formation that forces the drop of ink from the resistor through the nozzle and toward the print medium.
A problem that frequently arises with inks used in such thermal ink-jet applications is that the repeated heating of the resistor elements (and hence the ink) over several tens of millions of firings can cause breakdown of the ink, especially the organic components (in many cases, the dye molecule itself), with consequent fouling of the resistor element surface. This process is termed "kogation," strictly defined in the art as the buildup of residue (koga) on the resistor surface, and in many cases actually visible to the human eye upon microscopic examination of the resistor element. This buildup of koga degrades pen performance over time in that it acts as an insulating barrier between the ink and the resistor element, thereby reducing the efficiency of the vaporization process that takes place at the resistor and consequently reducing the volume of the ink droplet ejected from the print head onto the print medium. The problem ultimately manifests itself in the form of poor print quality on the print medium.
Thermal ink-jet inks typically contain a glycol, particularly ethylene glycol and/or diethylene glycol, as a humectant to reduce evaporation losses. Such evaporation leads to unacceptable crusting and clogging in the nozzles of the ink-jet pen, in part because the viscosity of the ink compositions increases substantially after some of the volatile components have evaporated.
It has recently been suggested that humectants such as ethylene glycol and diethylene glycol contribute to the problem of kogation. Although these compounds have been implicated in the kogation process, there is no conclusive evidence supporting the fact that the compounds themselves actually degrade at the resistor surface. When the actual koga is analyzed, it is found that the origin of the koga is predominantly the dye molecule and not the humectant (David J. Halko, "Kogation: A New Mechanism and Solution," 9th International Congress on Advances in Non-impact Printing Technologies, Oct. 4-8, 1993, Yokohama, Japan). This conclusion is based on the fact that the atomic percentages of the elements, usually carbon, sulfur, nitrogen, oxygen, and hydrogen found in the koga, are identical to that of the dye that is studied. It has also been shown that diethylene glycol will deposit polymeric materials (koga) at the resistor surface, as the ester linkage therein is thermally labile (P. L. Gender, et at., "Thermal Decomposition of Ethylene Glycol and Diethylene Glycol in Thermal Ink Jet Inks," 9th International Congress on Advances in Non-Impact Printing Technologies, Oct. 4-8, 1993, Yokohama, Japan). Thus, a theory is that, in essence, when incorporated into a thermal ink-jet ink, these humectants cause components of the ink to kogate. It seems clear that the buildup of koga at the resistor surface is the result of a thermo-chemical process wherein the dye and probably other components of the ink are transformed into some insoluble species (K. Shirota, et al., "Kogation of Inorganic Impurities in Bubble Jet Ink," 9th International Congress on Advances in Non-impact Printing Technologies, October 4-8, 1993, Yokohama, Japan; P. L. Gendler, et at., "IS&T," 8th International Congress on Advances in Non-impact Printing Technologies, 1992). Prior art workers have shown that the presence of Group I and Group II metal ions, particularly sodium, tend to increase the likelihood of kogation. Accordingly, much work has been directed toward removal of these deleterious ions. Since most inexpensive anionic dyes used in the industry are sodium salts of the corresponding basic dyes, research has focused on replacing the sodium cation with less active cations such as lithium, ammonium and various alkyl ammonium cations. Although elimination of deleterious ions such as sodium from the ink results in significant alleviation of kogation, the preparation procedures for the ink are so costly as to preclude widespread industry acceptance.
There have also been attempts to reduce kogation in thermal ink-jet inks by incorporating into the inks different compounds that ostensibly control kogation by various methods. See, for examples, U.S. Pat. Nos. 4,790,880; 5,108,501; and 5,169,437.
Despite the efforts to formulate thermal ink-jet inks that eliminate kogation, there remains the need for a thermal ink-jet ink that can be produced economically and that essentially eliminates kogation.